Because they can oxidize and decompose organic materials and some inorganic materials such as nitrogen oxides using light, which is cheap and has an extremely low environmental impact, photocatalytic materials have been applied in recent years to environmental cleanup, deodorization, soil prevention, sterilization and other applications, and a variety of photocatalytic materials are being developed and studied.
Well-known photocatalysts include titanium oxide, which is responsive to ultraviolet radiation, but for use in residential interiors and other environments with little ultraviolet radiation there is a need for photocatalytic materials that are responsive to visible light, and these are being studied and developed.
For example, Patent Document 1 discloses a photocatalytic material having visible light activity, in which the oxygen atom sites of titanium oxide crystals are partially substituted with nitrogen atoms.
In the photocatalytic material disclosed in this Patent Document 1, visible light activity is achieved by the formation of a new isolated energy level on the negative side of the valence band of titanium oxide when the oxygen atom sites of the titanium oxides crystals are partially substituted with nitrogen atoms. When the electrons at the isolated level are exposed to photons having energy equal to or greater than the bandgap energy between the isolated level and the conduction band, they are excited to the conduction band of titanium oxide, while holes are created in the isolated level, resulting in visible light activity.
However, the isolated level formed in this way on the negative side of the valence band of titanium oxide has low potential, so the oxidative power of the holes resulting from photoexcitation of electrons by exposure to visible light is low, and the movement of holes produced at the isolated level is also restricted, so reactivity with the substrate to be oxidized is low. As a result, the problem with the photocatalytic material disclosed in Patent Document 1 has been that it has visible light activity but low oxidative decomposition activity.
Visible light absorption can also be achieved by doping the titanium ion sites of titanium oxide with another metal ion, thereby shifting the conduction band minimum potential of titanium oxide to the positive side or forming an isolated energy level on the positive potential side of the conduction band minimum potential. However, when the conduction band minimum potential of titanium oxide is shifted to the positive side sufficiently to provide visible light absorption, or when an isolated level is formed on the positive potential side of the conduction band minimum potential, the potential of the shifted conduction band minimum or resulting isolated level is larger than the one-electron reduction potential of oxygen (−0.046 Vvs. SHE, pH=0), and the photoexcited electrons are no longer capable of one-electron reduction of oxygen. The photoexcited electrons recombine with the resulting holes and lose their oxidative decomposition activity, and because of this metal ion-doped titanium oxide has exhibited only very low oxidative decomposition activity.    Patent Document 1: Japanese Patent No. 3601532